tests: increase timeout for vm shutdown
tests: increase timeout for vm shutdown

start_standalone_with_cdrom_vm test essence is somewhere else, let not
fail it for just slow shutdown (LVM cleanup etc).

marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin